In the manufacture of semiconductor devices, conductive metal contacts and lines are deposited over dielectric layers, such as silicon oxide. The metal lines connect various devices to each other to form integrated circuits. Openings are made in the dielectric layer and filled with a conductive material so that contact between overlying lines and underlying devices can also be made.
As devices become smaller and more devices are made in a single semiconductor (silicon) wafer, the openings in the dielectric layer have a smaller diameter. These small diameter openings are difficult to fill, particularly using conventional sputter deposition processing.
FIG. 1 illustrates a conventional sputtering chamber. A vacuum chamber 10 includes a target 12 of the material to be sputtered and a substrate support 14. A source of DC power 13 is connected to the target 12. A pair of opposed magnets 16 and 18 are mounted on top of the target 12. A power source 20, such as a source of RF power, is connected to the substrate support 14. During sputter deposition, a substrate 22 is mounted on the substrate support 14. A gas inlet 19 permits gases to be passed into the chamber. Argon is generally used as a sputtering gas. The argon is ionized in the chamber and is attracted to the target 12 by the magnets 16, 18. The argon atoms strike the surface of the target and sputter off particles of target material which deposit on the substrate 22. If a material such as a nitride is to be formed on the substrate, nitrogen gas is also passed into the chamber where it is ionized and reacts with sputtered metal on the substrate.
Since sputtered particles are sputtered from a target in numerous random directions, comparatively few of the sputtered particles impact the target in a direction perpendicular to the substrate. Most of the sputtered particles thus impact the openings in the substrate at some other angle, causing the sputtered layer to build up along the sides and top of an opening rather than on the bottom. Further, as the aspect ratio of the opening becomes higher, it is even more difficult to cover the bottom of the opening. This top and side buildup creates an overhang over the opening, as shown in FIG. 2, which further prevents particles from depositing on the bottom of the opening. FIG. 2 illustrates an opening 100 partially filled with sputtered metal 110.
Aluminum has been widely used for the manufacture of conductive lines and contacts, but more recently copper has been tried. Copper is more conductive than aluminum, but has a similar problem as aluminum with respect to an underlying silicon substrate., i.e., at elevated temperatures the copper diffuses and reacts with other materials in the integrated circuit, and thus a barrier layer needs to be deposited between the conductive metal and the substrate.
Tantalum and tantalum nitride have been accepted as good barrier materials for copper to prevent the diffusion of copper into underlying layers. Tantalum can be deposited by sputtering, and, when sputtered in the presence of nitrogen, tantalum nitride is formed. Tantalum nitride is not as conductive as tantalum, and it has a tendency to peel off the underlying substrate, probably due to high stress in the tantalum nitride film. This peeling also has the disadvantage that it causes the formation of particles, which is always undesirable.
Tantalum is a better conductor and a better wetting agent, and thus is a good adhesive between copper and the underlying substrate. However, the use of tantalum alone has the disadvantage that it is not as good a barrier as tantalum nitride. Thus efforts to improve tantalum-containing barrier layers for copper have continued.